The invention relates to an anchoring member for attachment to a vertebra and for connection with a first rod having a first diameter or a second rod having a second smaller diameter. The anchoring member comprises a shaft and a head for connection to the rods, the head being connected to the shaft and having a generally U-shaped cross-section with a curved base and two legs laterally defining a first channel for receiving the first or second rods and a center axis extending between the legs perpendicular to the base, a fastening member for cooperation with the legs to fasten the rods within the first channel and a cylindrical recess in said head being concentric to the center axis and extending from the free end of the legs to the base of the first channel.
Such an anchoring member is known from document WO 94/08527. The radius of the base of the U-shaped channel, which is substantially equal to half of the distance between the two legs, is chosen as a function of the diameter of the rod to be received so that the rod can be placed into the channel without getting jammed. The use of rods having different diameters requires corresponding different anchoring members having their radius of the U-shaped base adapted to the diameter of the rod. This not only increases the costs of stock-keeping, but also renders the operation proper more difficult.
Document WO 93/11715 discloses an anchoring member having a shaft to be fastened to a bone and a head for connection with a rod. The head has a generally U-shaped cross-section and two free legs defining a first channel for receiving the rod therein. In order to allow the use of rods having different diameters an insert member is provided with a generally U-shaped cross-section and a second channel defined by the two free legs thereof for receiving a rod having a smaller diameter. The length of the insert member is greater than the length of the first channel. Projections are provided at both opposite ends of the insert member and the distance between the projections is substantially equal to the length of the first channel. After insertion the two projections engage the outside of the head and thereby prevent the insert member from being relatively displaced within the channel during the adjustment operation. This changes the outer contour of the head.
It is the object of the invention to provide an anchoring member with an insert member which is formed so that the above disadvantage is avoided.